In order to check to what degree an image of a camera is in focus, a prior art automatic focusing apparatus enhances the high frequency of image data which is photographed by the camera using a high-pass filter, determines a focus position on the basis of an index value obtained by averaging an absolute value of an output value of the high-pass filter or the square of the output value, and moves the objective lens to the determined focus position, thereby performing the automatic focusing operation (for example, refer to Japanese Published Patent Application No. Hei.10-042184, pages 3 to 7 and FIG. 1).
Particularly, a microscope for making a precise analysis, and the like are required to be properly focused on an observation target.
However, there is some cases where an influence of a noise such as a dust makes it difficult to automatically bring the observation target into focus properly in the above-described prior art.
Then, as an automatic focusing method for reducing the influence of the noise, in a case where an automatic focusing is performed using a transmitted light, suggested is a method in which a range in which the automatic focusing is performed is restricted to a range in which the observation target is present in a full field of view which is photographed by a camera, and only the image data in the range is used as an operation target for the automatic focusing.
FIG. 9 is a diagram illustrating a construction of a prior art automatic focusing apparatus.
An automatic focusing apparatus shown in FIG. 9 comprises a light source 901, a film 902, a lens unit 903, an image sensor 904, an A/D converter 905, an operation unit 906, a controller 907, a range designating unit 908, a stepping motor 909, and a sensor driving unit 910.
The light source 901 applies a light to the film 902.
The lens unit 903 performs zooming and focusing with respect to the image photographed on the film 902. The lens unit 903 is controlled as to movement by being driven by the stepping motor 909.
The image sensor 904 is a solid-state image sensor provided on an image forming surface in the lens unit 903, and reads a film image. The image sensor 904 is controlled as to movement by being driven by the sensor driving unit 910.
The A/D converter 905 subjects the image data read by the image sensor 904 to A/D conversion.
The operation unit 906 performs an operation to obtain a focus value for the A/D converted image data.
The controller 907 controls the stepping motor 909 and the sensor driving unit 910 on the basis of the output from the range designating unit 908 and the output from the operation unit 906.
The range designating unit 908 previously designates a range of image.
An operation of the automatic focusing apparatus so constructed will be described.
Initially, the light source 901 applies a light to the film 902, and the image sensor 904 reads a transmitted light through the lens unit 903.
Then, the A/D converter 905 quantizes the image data outputted from the image sensor 904. The controller 907 controls the stepping motor 909 for driving the lens unit 903 and the sensor driving unit 910 for driving the image sensor 904 on the basis of the output signal, from the operation unit 906 and the output signal from the range designating unit 908.
The automatic focusing is executed using image data in a range previously designated by the range designating unit 908, in which an effective image is more likely to be present and thereby an optimal focus position is obtained by the operation unit 906 (for example, refer to Japanese Published Patent Application No. 2001-154269, page 18 and FIG. 7).
In the above-described prior art automatic focusing apparatus, however, there is a problem that since a range in which the automatic focusing is performed is restricted to a periphery of an observation target, in a case where the observation target and dust and the like are mixedly present, it is difficult to set the range, and thereby it is impossible to properly perform automatic focusing even when the observation target is sufficiently larger than the dust and the like.